It is well known in the art that the dehydrated products offer numerous advantages with respect to the original natural products, such as a more prolonged preservation of quality, more efficient transportation and storage without refrigeration, less storage space per unit of solids, and considerable decrease in transportation costs per unit of solids.
In the particular case of tomatoes, the obtention of a granulated dehydrated product constitutes a very important processing option, inasmuch as this product has acquired a great demand at the international level. The powdered dehydrated tomato shows numerous advantages with respect to the paste obtained from tomato, due to the dehydration degree achieved, which permits the handling of much larger amounts of solids per unit of volume, as well as the removal of any microbial growth and other chemical or biochemical processes that damage the quality of the product. All of the above is highly advantageous for the manufacturer and the consumer from the economical standpoint, because the manufacturer will be able to offer, and the consumer will be able to acquire, a product of excellent quality at a reasonable price.
In later years, due to the high degree of technicalization of this process, it has been possible to obtain a dry material having a very good quality, which gradually acquires more demand from companies engaged in the preparation of dehydrated or semi-cooked foods, such as vegetable soup, dry pastes, juices, sauces, infant food, salad dressings and seasoners.
The most adequate drying method for a food product is determined by the requirements of quality and by economical factors, whereby it has been considered as indispensable to select the method which, being the most economical one, will furnish the desired characteristics of quality in the product obtained.
Many different drying processes are known in the prior art for dehydrating tomatoes and other vegetables, some of which will be described hereinbelow, but none of which has been able to produce a dehydrated product that may preserve the necessary physical and sensorial or organoleptic qualities of flavor, color, aroma and the like, or which are only able to produce a product that retains such characteristics only through very costly operations that increase the cost of the dehydrate product.
Among the many drying processes for tomatoes known in the prior art, the direct drying of tomato slices may be mentioned, in which the tomato fruits are washed and mechanically sliced by the action of very sharp knives which avoid tearing the fruits. The slices are placed in pans containing mineral oil to avoid sticking of the product and the drying is carried out in tunnels until a moisture of approximately 4% is obtained. This drying operation has also been effected by means of the use of solar energy. The product is milled in hammer mills and quickly put in moisture-proof containers in order to avoid absorption of water due to the high hygroscopicity of the powdered material.
It is to be stated that this simple drying process which has been used in the prior art, does not produce a product which may be reconstituted to its original size and shape, after rehydration thereof, but that the dehydrated tomato particles, after rehydration, remain collapsed, even after being soaked in water and cooked. This product, therefore, has been restricted for use in mixtures of dry soups, dehydrated mixed vegetables, canned soups, "chili con carne" and the like. Also, in order to obtain an acceptable product by means of this method, it is necessary to use a variety of tomatoes having a bright red color, a high solids content, a thick skin and a high level of pectin.
Many other drying processes for tomatoes are known in the art, all of which require the carrying out of several previous operations that are common to all processes and that therefore will be described hereinbelow as admitted prior art.
The tomato fruits are selected, in order to remove those presenting an inadequate ripening degree, an intensive contamination by molds or insects and physical damage, inter alia. The selected tomato fruits are then ruptured and milled, pulped or disintegrated, in order to form a juice or pulp of the fruit, from which the skin and the seeds are removed, and the pulping operation is continued until the particle size is reduced to the desired dimensions. The breaking of the tomato may be made in hot or in cold, but the hot breaking, comprising heating the fruit to a temperature of at least 104.degree. C. for at least 15 seconds, is preferred in view of the fact that by this means the deactivation of the pectinolytic enzymes (pectinesterase and polygalacturonase) is readily accomplished, thus avoiding hydrolysis of the pectic material. The natural pectins of the tomato account for the suspension of the solids of the juice, after the extraction thereof.
The so called cold breaking of the tomato has also been used in the processes of the prior art, and it has been effected by means of the heating of the tomato fruits to facilitate the release of the skin, thus facilitating the extraction of the pulp and consequently increasing the yield of the process. The enzymes contained in the fruit hydrolyze the pectic substances, thus permitting the resulting juice to be concentrated to levels above 30.degree. Bx. However, if the product is handled at lower concentrations, its rheological characteristics are damaged, whereby it is normally necessary in this process to add thickening agents.
The thus obtained pulp is concentrated either at ambient pressure or preferably at reduced pressure, in view of the fact that the vacuum evaporation of the juice permits the obtention of a product having a better quality by decreasing the thermal damage. The degree of evaporation used in the prior art has depended on the type of breaking and drying methods to which the product is to be subjected.
The above described stages have been common to all existing processes of dehydrating tomatoes in the prior art, regardless of the drying operation used, and in this particular respect it is to be stated that many different drying processes have been used for dehydrating tomato in accordance with the prior art.
One of the most popular drying methods used in the prior art is the lyophilization of the concentrate obtained in accordance with the above described stages, which comprises freezing the concentrate and then drying the same under high vacuum conditions in order to allow the water in the solid state to sublimate at low temperature. That is, the water is removed from the material by passing directly from the solid to the gaseous state, without passing through the liquid state. In this manner, the solids are maintained in its initial structure, without losing its shape and original size and thus, the structure of the lyophilized particles is very porous, preserving the spaces originally occupied by the water without collapsing, which favors the fast reconstitution of the product when rehydrated.
Although this process has been used at an industrial level in the prior art, it is to be stated that, while the process is capable of obtaining a dehydrated product having excellent characteristics, the conditions necessary to carry out this type of drying operation involve very low temperatures and a high vacuum, for which a very costly equipment is required and for which considerable periods of time are consumed in the drying operation, thus rendering said process relatively costly whereby the products obtained must be sold at a higher price.
One other process which is presently being used in the dehydration of tomatoes, as an attempt to overcome the problems of the lyophilization process as to cost, is the so called expansion drying process, which was introduced in the industry during the '60s, and which comprises an expansion-dehydration by the use of hot air, with which the workers in the art were trying to obtain a product similar to that obtained by means of the lyophilization process.
The expansion drying process generally requires absolute pressures of the order of 2.6 mm of mercury for carrying out the dehydration, and the temperature of the material is usually between 60 and 70.degree. C., although in the initial stages said temperature may be increased up to 90.degree. C. The temperature of the product may vary from low temperatures in the initial stage of the cycle, up to the temperature of the drier in the final stages. The drying times are of from 90 minutes to 4 to 5 hours in order to obtain a moisture content of approximately 3%, and the dried product is cooled before breaking the vacuum, so as to avoid the expanded structure to be collapsed, due to the fact that the thus obtained product is frequently thermoplastic.
The expansion drying differs from lyophilization in two main aspects, namely, the use of temperatures above the freeze point, which permits that, during the drying operation, a higher vapor pressure be obtained and, consequently, a lower drying time may be rendered possible. The other difference is the degree of expansion of the product, which is higher than with lyophilization in view of the fact that in the latter no change in volume is produced. The expanded structure furnishes to the dry material characteristics of quick reconstitution by rehydration and the high ratio of area/volume permits a good heat transfer and steam absorption, although if the expansion is higher than desirable, the heat transfer through the expanded structure is reduced and some wet or frozen areas may remain at the center of the mass.
Although this process has been used for dehydrating a multiplicity of vegetable materials, it has been found that when applied to the tomato concentrate obtained by means of the prior stages defined above, the tomato mass cannot be easily expanded without the previous incorporation of air. The non-aerated material is very slowly dried, with the obtention of a hard vitreous product having a volume similar to that of the concentrate and providing a very slow reconstitution. When air is incorporated into the concentrate in order to provide a bulk density of from 0.9 to 1.0 g/ml., the material can be expanded, dried and reconstituted in a satisfactory manner. However, in the most modern expansion drying processes applied to tomato concentrates, the product must be removed from the drier at an intermediate time during the drying operation, expanded outside the drier and returned to the drier, which causes a considerable problem of controlling the drying operation as a whole, whereby this process has not resulted in a suitable process for dehydrating tomatoes, and has been rather used for the dehydration of other different products such as mushrooms an the like. On the other hand, the cost of the provision of the vacuum necessary for the expansion, also renders this process relatively costly.
One other very popular drying process used for the dehydration of vegetable products in the spray drying process, in which the material in very small droplets is flash-dried in contact with a hot gaseous stream, the dry particles being carried out or entrained in said gaseous stream. The small size of the particles permits a very fast drying and the residence time of the material in the drier is of only a few seconds at most. The dried product is recovered from the gaseous stream by means of a suitable separation equipment and the dry powder is recovered from the system and cooled in a continuous manner, so as to avoid any thermal damage through prolonged contact with the hot gases.
When each droplet of the liquid product is being dried, the temperature of the product is maintained at the level of the wet bulb temperature of the drying gases, whereby the relatively high dry bulb temperature of said drying gases may be tolerated, thus causing a minimum thermal damage to the thermally sensitive components of the concentrate. On the other hand, the rate of the reactions that may damage the food products decreases when the moisture level is also decreased and thus, in the final stages of the drying process, where the temperature may increase beyond that of the wet bulb, no severe damage is caused to the product.
The spray dried particles are normally hollow spehres or fragments of spheres, and this shape of the final product is responsible for the excellent rehydration properties of the spray dried powders. This process is the most popular for use in the dehydration of tomatoes, and the dry product is presently commercialized with or without the addition of certain additives, and is used in the manufacture of juices, mashed tomato, pastes, soups and sauces. The tomato solids obtained by the spry drying process show a multiplicity of advantages such as low cost, and remarkable ease of handling, packaging and transportation and admixing.
Of all the above described techniques for the dehydration of tomatoes, the spray drying method would seem to be the most suitable process applied in prior art installations for the obtention of a high quality powder at a reasonable cost. However, the driers used for these purposes require a very special design of the drying chamber, which may permit the droplet which is being dehydrated to be dried without suffering any overheating. On the other hand, special handling and packaging techniques are required in order to avoid rehydration of the powder in view of its highly hygroscopic character.
Several types of designs of spray driers for this purpose are available in the market, both for carrying out the spray drying operation under conditions of moderate temperatures (75.degree. to 90.degree. C.) and using air as the drying gas, which permits to have a high production speed, or for carrying out the spray drying operation by the use of low temperature conditions (25.degree.-30.degree. C.) and by the use of humidified air. However, all these spray driers for use in the dehydration of tomatoes require extremely tall drying chambers, and therefore considerable heights in the apparatus to be erected, with the consequent increase in costs of installation of the plants.
For instance, applicant has information about a spray drier of the low temperature type which has been built with a height of approximately 80 meters and a diameter of approximately 16 meters, using a rotary spraying apparatus of low velocity, in order to obtain yields of about 200 kg. per hour of dehydrated powder. However, these driers must be normally operated with pastes obtained by the method of cold breaking and, although the powder obtained presents good fluidity and low tendency to agglomerate, when reconstituted it lacks body and presents a fast sedimentation, thus requiring the addition of thickeners in order to compete with other types of powder obtained by hot breaking of the material.
Examples of the above type of spray driers have existed for some time. It may be mentioned, for instance, that in 1967 Tree Valley Growers, of Modesto, Calif., started operations in the area of tomato dehydration, through the use of a Danish equipment formed by a tower about 30 meters high and with a capacity of production of 1,356 metric tons of dehydrated food products per year.
One other example exists in Spain where spray dried tomato powder has been prepared, with the addition of citric and ascorbic acids to compensate for the losses of vitamin C caused by the heat applied in the previous concentration step which is necessary for producing a paste of the necessary consistency for use within the spray drier itself.
The above type of drying operation for the obtention of dehydrated tomatoes, although very popular in the field, may be regarded as involving extremely high costs of installation of equipment, as well as difficult controls in the operation.
During the 1960s, the dehydration of vegetable products on a foam mat gained great attention. Initially the workers in the art thought that this type of drying would show a great number of advantages over other dehydration systems, inasmuch as in the particular case of the foam mat drying operations the loss of water was very fast, due to the movement, by capillarity, of the liquid fraction, which is separated from the bubbles of the foam and thus, by providing a porous structure, the rehydration of the product was almost instantaneous.
The key for this type of drying process was based on the formation of a stable foam, by the addition of foaming products such as soy protein, albumin, fatty acid esters and saccharose, as well as glyceryl monostearate, among other foaming products. This type of drying was used for instance by Patterton Canning Co., by transforming the tomato pulp into a foam through the addition of glyceryl monostearate, added to the pulp in a 10% dispersion in warm water. The foam was obtained by stirring and was deposited on drying pans, where by means of air streams, pores were opened to facilitate the dehydration by increasing the drying area. The air was fed to the drier at temperatures of about 93.degree. C. and at a high velocity of about 100 to 130 meters per second, in a countercurrent direction. The times of drying were of from 15 to 18 minutes, depending on the characteristics of the product and on the conditions used.
Although this type of processes utilizing foam mat drying were considered at the time as capable of obtaining a product with improved flavor and color as compared to the spray drying and drum drying operations, the process had the disadvantages that the packaging of the obtained products had to be done in areas with conditioned atmosphere having relative moisture levels of 50% or less, in order to avoid the absorption of moisture due to the hygroscopicity characteristics thereof.
Also, in order to have a reasonable shelf life, a packaging material impermeable to gases and to ambient moisture was necessary and the product had to be packed in inert atmospheres in order to avoid damaging changes caused by oxygen on the components of the tomato.
Moreover, the reconstitution of the product by the addition of water furnished a product with a discolored aspect due to the retention or air bubbles, which rendered the product less red than the natural product. On the other hand, it was found out that during the concentration of the tomato juice, as well as in the first stages of the drying operation, a great proportion of the original aroma was lost through the volatilization of large amounts of volatile low molecular weight products, which damaged the organoleptic characteristics of the reconstituted product. Also, the evaluation of the shelf life of the product obtained by this process, proved that during a prolonged storage of the tomato powder, unpleasant flavors were developed, these being affected by the temperature of storage, and it was determined that the components that suffered the most important changes were the free aminoacids, even at storage temperatures as low as 0.degree. C. The changes of the concentration of aminoacids were observed in the paste, in the dried product and in the stored product, and said changes were also associated with a considerable loss of the color characteristics, since the product was darker than the products obtained by other processes.
One other process that was used during the l940s was the drum drying process in order to produce dehydrated tomato flakes having a reasonable quality, which process was considerably improved during the l960s, by modifying the drum driers with the aim of improving the flavor and color characteristics as well as the stability of the powdered dehydrated tomato. These modifications included the incorporation of an air stream in countercurrent with the direction of rotation of the drums, as well as the adaptation of the collecting zones by providing them with an atmosphere having a relative humidity from 15 to 20%.
This process, however, never accomplished the goal of truly dehydrating tomatoes, because it was indispensable to leave moistures of at least 7% in the product, thus requiring a further drying step for more than 24 hours with air at 20.degree. C. without movement, inasmuch as otherwise the product was deteriorated as regards the color and the flavor.
Other known processes for dehydrating tomato solids are based on a fractional drying process, by means of which the juice, before concentration thereof, was centrifugated in order to obtain two separate fractions, a solids fraction and a serum fraction, which latter fraction was concentrated under vacuum, to be thereafter dried also under vacuum to moisture levels of about 3%. The solids fraction was also dehydrated under vacuum and finally the two fractions already dried were admixed and milled in order to obtain a product having good characteristics of reconstitution. This type of processes, however, require a strict control in the centrifugation and in the drying of the separate fractions, with the consequent increases in the costs of production of the product utilizing this method.
Many other processes are known in the art, and have been patented in various countries of the world, such as the process described in Japanese Patent Publication 9,272/62, which refers to a process for dehydrating tomato derivatives such as catsup, wherein carboxymethylcellulose is incorporated to the material under dehydration, and the dehydration is effected either by spray drying or by lyophilization.
U.S. Pat. No. 3,290,788 patented Dec. 13, 1966, describes a process for drying organic materials in liquid solution which is applicable in general to any organic liquid such as milk, fruit juices, vegetables, etc., and in which the dehydration of said organic materials is effected by lyophilization using very special conditions, such as the application of radiation to the frozen product under vacuum, by using adequate wavelengths in order to sublimate the water without furnishing any heat to the product.
U.S. Pat. No. 3,323,923 patented June 6, 1967, describes an improvement in the foam mat drying method for dehydrating food materials, in which the improvement resides on the use of different stabilizing agents for the foam, such as esters derived from monoglycerides of higher fatty acids, polyglycerines and polyglycerine esters. Other than the use of this different stabilizing agents for the foam, the process described in this patent is exactly the same that already described in the admitted prior art.
U.S. Pat. No. 3,567,469 to Corn Products Co., describes a process for removing water from thermosensitive food products, which comprises the formation of a stable dispersion of the finely divided product, to which a water repellent was added, in order to permit the moisture to be evaporated from the films formed, in a continuous and fast manner by de use of evaporating systems. The time of drying varies from 2 to 3 seconds up to 10 minutes, and the product may reach a maximum temperature of about 65.degree. C. and a final moisture content of about 4%. This process, however, incorporates foreign materials to the product, such as the necessary water repellent for the formation of the films of water, which may be regarded as a disadvantage in view of the fact that the flavor and the color of the finished product are damaged to a certain extent.
Swiss Patent 464,666, published Dec. 13, 1967, describes the obtention of instantaneous powders of different organic products, including tomato, by means of a special type of dehydration which is effected by placing the material in layers over a solid porous material, and thereafter forcing a drying gas through the pores of the solid material upwardly to traverse the layer of the product for a time sufficient for effecting the dehydration of the latter. This process, however, shows the disadvantage that a very special type of porous solid material must be used, and the necessity of using an inert gas such as nitrogen or carbon dioxide when treating materials which may be damaged through oxidation. Only in the case of non-oxidatable products may purified air be used. Food products such as tomatoes are highly damaged through oxidation, the necessity of using an insert gas for the drying operation is of course a problem that can only be solved through costly installations for producing such an inert gas, and on the other hand the drying of the products may need times as long as 200 hours, which renders said process rather impractical.
German Patent 1,924,490 published Nov. 20, 1969 describes a drum drying process for the obtention of dehydrated tomatoes, wherein a traditionally known drum drying operation is used in order to obtain tomato flakes, using for the obtention of a sufficiently well formed flake certain amounts of flour or starches to increase the consistency of the mixture to be dried on the drum. The addition of these foreign products such as starches an flour to the original product, however, considerably affects the final properties of the dehydrated product.
German Patent publication 1,943,798 published Mar. 5, 1970, in turn, describes a process for dehydrating fruits and vegetables suspended in a liquid medium, which process is characterized by adding to the fluid medium a substance which is suitable for temporarily occupying the porosity of the cells of the fruits or other vegetables, and thus maintain the cells without any collapsing during the drying process so as to maintain the porosity by means of the liquid in which the cells are suspended. The drying process, however, is normally effected under vacuum and with antioxidants, chloroform or sulfurous acid is being added to the product in order to avoid oxidation thereof. This process, in a way, is somewhat complicated and has not gained any commercial penetration in the market.
Japanese publication No. 18 258/70, describes a fractionation process for obtaining a powdered dehydrated tomato product, in which the tomato pulp is fractionated in order to separate the serum from the remaining solids, and the serum is admixed with monoglycerides, foamed and dehydrated by lyophilization. The remaining solids are also lyophilized and the resultant products are admixed in order to obtain a dehydrated product in less time than when the same lyophilization process is used with the integral product. This publication, however, may be regarded to be included within the fractionating and lyophilized processes described above, with the consequent disadvantages already mentioned.
Japanese publication No. 16 619/73, also describes a lyophilization process for dehydrating various product including tomatoes, and incorporates anti-oxidants an flavor potentiators for avoiding oxidation of the material as well as preservation of the original flavor.
U.S. Pat. No. 4,031,266 patented June 21, 1977 and assigned to General Foods Corp., describes a process for drying an admixture of tomato paste and unmodified starches by the use of a drum drier in order to obtain flakes which are thereafter milled to produce the final product. It is necessary to add certain additives for recovering the original flavor of the tomato, which is a definite disadvantage over the absence of said additives, and the drum drying process used has the disadvantages already discussed above.
Dutch Patent No. 7,601,204 published Aug. 9, 1977 refers to the obtention of animal or plant material concentrates by separating the liquid fraction from the original product in order to reduce the time of evaporation, using less energy and causing less thermal damage to the product. This patent, however, merely refers to the obtention of the paste material and is not related to the drying process itself, inasmuch as it merely mentions that the product thus obtained by the above fractionation of the material, may be used for the obtention of powders including tomato powder.
U.S.S.R. Patent No. 581,919 on Nov. 30, 1977 describes a method for the obtention of tomato powder having characteristics of fast reconstitution, said process comprising the spray drying of tomato pulp admixed with a predetermined proportion of starch, in order to obtain agglomerates that are thereafter thoroughly dried with air. The process described in this patent, however, includes two drying stages, one which is effected by spray drying of the tomato pulp with the addition of starch and one other which is effected by means of air for drying the agglomerates produced in the first drying stage, and with the incorporation of additional starch in this second drying stage. The addition of starch and the rather complicated drying process used, renders this process uncapable of large capacities.
Italian Patent No. 1,045,453 of Nov. 18, 1975, describes a process for the preparation of a powdered tomato product, which uses a drum drying process wherein the initial product is formulated by the admixing of tomato juice with a thickener product and heating on the drum to a temperature of about 113.degree. C. in an atmosphere of low relative humidity. This process, therefore, shows the same disadvantages already discussed above for the drum drying type of drying process.
East German Patent No. 201,847 of Aug. 17, 1983, describes a process for drying tomatoes, which may use any drying process, provided that the tomato concentrate having approximately 30% of solids, is admixed with a relatively high proportion of a thickening agent such as dextrins and with a certain proportion of common salt, and homogenizing the mixture before subjecting the same to the drying process. It is alleged that the dried product obtained, upon reconstitution by rehydration, preserves the characteristic flavor of tomato and the color thereof, and the salt is alleged to provide a more grainy characteristic to said powder. However, the addition of these additives in relatively large proportions to the tomato concentrate may be regarded as a definite disadvantage over the processes that do not use said type of additives.
To the knowledge of applicant and after having effected a relatively large number of tests, by using the processes of the prior art described above, it has been concluded that the main parameters associated with the decrease in the quality of the powdered dehydrated tomato products, are: a) the abuse to which the juice is subjected during the processing thereof in order to obtain the paste, b) the existence of occluded air in the paste which causes oxidation problems in the further steps of the process, and c) the particle size of the final product obtained by said processes, which given its hygroscopicity, tends to promote the formation of agglomerates which gradually suffer damages such as discoloration, as well as loss of free flow, taste and aroma.
On the other hand, a relatively large proportion of the products obtained by the prior art methods described above, either contain or are admixed with starches, dextrins and other filling agents, as well as anti-caking agents, thickening agents and the like, which also damage the characteristics of flavor, color and aroma and therefore render the thus obtained products inadequate to be compared with completely natural products. Some other processes, particularly when the fruit is ruptured by the cold breaking method, require the addition of considerable amounts of thickeners, in view of the fact that by the cold breaking process, the pectinolytic enzymes are inactivated and therefore the rehydrated product lacks body and consistency unless certain amounts of thickeners are added.
The only processes that have been able to avoid oxidation of the product during the drying step, are those utilizing lyophilization, which is a relatively costly and slow method, that has not gained sufficient importance in the market. All other processes, due to the existence of oxygen in the paste and due to the fact that they are carried out normally at relatively high temperatures, cause considerable scorching and oxidation of the product, which also tends to discolor the same as well as to cause a loss of flavor and aroma in the reconstituted product, thus producing products that are not favorably compared with natural products These oxidation processes, obviously, also damage the vitamin C content of the products, beside the obvious damage of the organoleptic and rheological properties of the thus obtained dehydrated tomato products.